recombination

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recombination,

process of "shuffling" of genesgene,
the structural unit of inheritance in living organisms. A gene is, in essence, a segment of DNA that has a particular purpose, i.e., that codes for (contains the chemical information necessary for the creation of) a specific enzyme or other protein.
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 by which new combinations can be generated. In recombination through sexual reproductionreproduction,
capacity of all living systems to give rise to new systems similar to themselves. The term reproduction may refer to this power of self-duplication of a single cell or a multicellular animal or plant organism.
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, the offspring's complete set of genes differs from that of either parent, being rather a combination of genes from both parents. In recombination by crossing overcrossing over,
process in genetics by which the two chromosomes of a homologous pair exchange equal segments with each other. Crossing over occurs in the first division of meiosis. At that stage each chromosome has replicated into two strands called sister chromatids.
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, alleles of genes are exchanged between homologous chromosomes during meiosismeiosis
, process of nuclear division in a living cell by which the number of chromosomes is reduced to half the original number. Meiosis occurs only in the process of gametogenesis, i.e., when the gametes, or sex cells (ovum and sperm), are being formed.
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. This exchange results in the generation of new combinations of alleles on segments of chromosomes, counteracting the tendency of linked genes, i.e., genes on the same chromosome, to be always transmitted as a group. Various mechanisms for introducing new genetic material have been discovered in bacteria. These mechanisms have been used extensively to study gene structure and function. In transformation, a fragment of free deoxyribonucleic acid (DNA) is inserted in a recipient bacterium (see nucleic acidnucleic acid,
any of a group of organic substances found in the chromosomes of living cells and viruses that play a central role in the storage and replication of hereditary information and in the expression of this information through protein synthesis.
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). The free DNA fragment comes from the chromosomechromosome
, structural carrier of hereditary characteristics, found in the nucleus of every cell and so named for its readiness to absorb dyes. The term chromosome
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 of a bacterial cell that has been lysed, or dissolved. In transduction, genetic material is transferred from one bacterium to another by a carrier virus. When a virus enters a bacterium, its DNA can be inserted into the bacterial chromosome, reproducing along with the host chromosome in cell division. Subsequently, sometimes many bacterial generations later, the viral genetic material may detach from the bacterial chromosome, taking some of the bacterial chromosomal material along with it. The bacterium then lyses, the viral particle enters a new bacterium, and the viral particle, together with the bacterial genes it carries, is inserted into the chromosome of the new host. In conjugation, which occurs between bacteria of the same species and also between some bacteria of different species, either an entire chromosome or a part of one is transferred from a bacterium of a donor-mating strain to a bacterium of a recipient-mating strain. Donor strains, denoted male, contain a sex-factor particle composed of a nucleic acid; recipient, or female, strains lack the particle (see episomeepisome
, unit of genetic material composed of a series of genes that sometimes has an independent existence in a host cell and at other times is integrated into a chromosome of the cell, replicating itself along with the chromosome. Episomes have been studied in bacteria.
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). Conjugation has been used to construct genetic maps, i.e., the ordering of genes along a chromosome. Evidence from transformation experiments was used to support the idea that DNA was the genetic material.

Recombination (genetics)

The formation of new genetic sequences by piecing together segments of previously existing ones. Recombination often follows deoxyribonucleic acid (DNA) transfer in bacteria and, in higher organisms, is a regular feature of sexual reproduction. See Deoxyribonucleic acid (DNA), Reproduction (animal), Reproduction (plant)

The fact that recombinants occur in sexual reproduction is due to reciprocal exchanges between chromosomes (crossing over) that take place in the first meiotic division. See Crossing-over (genetics)

Crossing-over between homologous chromosome pairs can also occur during the prophase of mitotic nuclear division. The frequency is very much lower than in meiosis, presumably because the mitotic cell does not form the synaptic apparatus for efficient pairing of homologs. See Mitosis

Recombination was once thought to occur only between genes, never within them. Indeed, the supposed indivisibility of the gene was regarded as one of its defining features, the other being that it was a single unit of function. However, examination of very large progenies shows that, in all organisms studied, nearly all functionally allelic mutations of independent origin can recombine with each other to give nonmutant products, generally at frequencies ranging from a few percent (the exceptionally high frequency found in Saccharomyces) down to 0.001% or less. Recombination within genes is most frequently nonreciprocal.

Bacteria have no sexual reproduction in the true sense, but many or most of them are capable of transferring fragments of DNA from cell to cell by one of three mechanisms. (1) Fragments of the bacterial genome can become joined to plasmid DNA and transferred by cell conjugation. (2) Genomic fragments can be carried from cell to cell in the infective coats of bacterial viruses (phages), a process called transduction. (3) Many bacteria have the capacity to assimilate fragments of DNA from solution and so may acquire genes from disrupted cells. Fragments of DNA acquired by any of these methods can be integrated into the DNA of the genome in place of homologous sequences previously present. Homologous integration in bacteria is similar in its nonreciprocal nature to recombination within genes of eukaryotic organisms. See Bacterial genetics, Bacteriophage, Transduction (bacteria)

Bacteriophages, plasmids, bacteria, and unicellular eukaryotes provide many examples of differentiation through controlled and site-specific recombination of DNA segments. In vertebrates, a controlled series of deletions leads to the generation of the great diversity of gene sequences encoding the antibodies and T-cell receptors necessary for immune defense against pathogens. All these processes depend upon interaction and recombination between specific DNA sequences, catalyzed by site-specific recombinase enzymes. The molecular mechanisms may have some similarities with those responsible for general meiotic recombination, except that the latter does not depend on any specific sequence, only on similarity (homology) of the sequence recombined.

Techniques have been devised for the artificial transfer of DNA fragments from any source into cells of many different species, thus conferring new properties upon them (transformation). In bacteria and the yeast S. cerevisiae, integration of such DNA into the genome requires substantial sequence similarity between incoming DNA and the recipient site. However, cells of other fungi, higher plants, and animals are able to integrate foreign DNA into their chromosomes with little or no sequence similarity. These organisms appear to have some system that recombines the free ends of DNA fragments into chromosomes regardless of their sequences. It may have something in common with the mechanism, equally obscure, whereby broken ends of chromosomes can heal by nonspecific mutual joining. See Transformation (bacteria)

The science of genetics has been revolutionized by the development of techniques using isolated cells for specific cleaving and rejoining of DNA segments and the introduction of the reconstructed molecules into living cells. This artificial recombination depends on the use of site-specific endonucleases (restriction enzymes) and DNA ligase. See Gene, Gene action, Genetic engineering, Genetics, Restriction enzyme

Recombination

 

in genetics, the redistribution of the genetic material of parents in their offspring, which results in the hereditary combinative variation of living organisms. When genes are not linked and occur in different chromosomes, redistribution may happen during the random combination of chromosomes during meiosis. When genes are linked, redistribution usually occurs during the crossing-over of chromosomes.

Recombination is a universal biological mechanism that is characteristic of all living systems, from viruses to higher plants and animals and man. The distinguishing characteristics of the recombination process depend on the level of organization of a living system. The process is simplest in viruses: during the joint infection of a cell by related viruses that differ in one or more characteristics, both the initial virus particles and the recom-bined particles that develop at a particular mean frequency and have new gene combinations are observed after the lysis of the cell. In bacteria several processes result in recombination, namely, conjugation, transformation, and transduction. In conjugation two bacterial cells are joined by a protoplasmic bridge, and a chromosome is transmitted from the donor cell to the recipient cell. Subsequently, certain areas of the recipient’s chromosome are replaced by corresponding fragments of the donor. Transformation is the transmission of characteristics by molecules of DNA that penetrate through the cell membrane from the environment. Transduction is the transmittance of genetic matter from the donor bacterium to the recipient bacterium, which is accomplished by a bacteriophage.

In higher organisms recombination occurs in meiosis during the formation of gametes. Homologous chromosomes approach each other and with great precision line up side by side (synapsis); this is followed by the breakage of the chromosomes at strictly homologous points and the crosswise reunification of their fragments (crossing-over). The result of recombination is revealed in the new combinations of characteristics in offspring. The probability of crossing-over between two points on the same chromosome is approximately proportional to the physical distance between the points. This makes it possible, on the basis of experimental data on recombination, to construct genetic maps of chromosomes, which graphically distribute genes in a definite scale and in a linear arrangement that corresponds to the location of the genes in the chromosomes.

The molecular mechanism of recombination has not been studied in detail; however, it has been established that the enzyme systems that ensure recombination also participate in such an important process as the repair of damage to genetic material. Endonuclease, the enzyme that brings about the primary breakages in DNA chains, activates after synapsis. In many organisms these breakages apparently occur in structurally determined areas called recombiners. Subsequently, the exchange of double and single DNA chains occurs, and special synthetic enzymes—DNA polymerases—fill the gaps in the chains, while the enzyme ligase seals the last covalent bonds. These enzymes have been isolated and studied only in some bacteria, but this research has brought closer the creation of a recombination model in vitro (in the test tube).

One of the most important consequences of recombination is the formation of reciprocal offspring; that is, in the presence of two allelic forms of genes AB and ab, two products of recombination—Ab and aB—must be obtained in equal numbers. Reciprocity is observed when recombination occurs between sufficiently distant points on the chromosome. During intragenic recombination this pattern is often disturbed; this phenomenon has been studied mainly in lower fungi, and is called gene conversion.

Recombination is important in evolution because it is often not individual mutations that are favorable to the organism but rather combinations of individual mutations. However, the simultaneous development of a favorable combination of two mutations in a cell is not likely. Recombination results in a combination of mutations that belong to two independent organisms and thus accelerates the evolutionary process.

REFERENCES

Kushev, V. V. Mekhanizmy geneticheskoi rekombinatsii. Leningrad, 1971.
Elementarnye protsessy genetiki. Leningrad, 1973.

S. E. BRESLER

recombination

[‚rē‚käm·bə′nā·shən]
(genetics)
The occurrence of gene combinations in the progeny that differ from those of the parents as a result of independent assortment and crossing-over.
The production of genetic information in which some elements of one line of descent are replaced or added to by those of another line.
(physics)
The combination and resultant neutralization of particles or objects having unlike charges, such as a hole and an electron or a positive ion and a negative ion.